This invention relates generally to a fluid supply system, and more specifically to a pumping system that delivers a controlled mixture of two or more constituents of liquid, being especially adapted for use in a liquid chromatograph as a mixing pump supplying solvent to a separation column.
The requirements of a mixing pump system in a liquid chromatograph are very stringent. Liquid flow into its separation column must have a substantially uniform flow rate and liquid pressure. Further, in gradient chromatography, the composition of the liquid so supplied must be gradually altered over time. Separate liquid constituents are mixed together in varying relative concentrations over time by the mixing pump system.
There are basically two categories of liquid mixing pumps that have previously been used for such an application. One category is known as a high pressure mixing pump system, and the other category is known as a low pressure mixing pump system.
In the high pressure mixing pump system, each liquid constituent, or component, to be mixed is connected from a reservoir of that component to an input of its own liquid pump. That is, each component is put under pressure by its own dedicated pump, and the outputs of each of the component pumps are joined together to form the output of the mixing pump system that is connected to the separation column of a liquid chromatograph, in that particular application. The flow rate of each pump is adjusted to provide the right proportions of the liquid components in its output, and to control the total flow velocity in that output. A usual application requires the proportions of the components to change over time, termed "gradient" flow, which requires the flow rates of the individual pumps to be appropriately changed over time to accomplish this. For example, if two components are being supplied in the mixing pump output with the percentage of one component increasing over time and that of the other component decreasing, the respective pumps for these components have their flow rates respectively increased and decreased over time in order to alter the balance of component concentrations without changing the total output flow rate.
A significant disadvantage of high pressure mixing systems is that a separate pump is required for each solvent or other liquid component that is being mixed. In some cases, more than two such components must be mixed. In the liquid chromatograph supply application particularly, the pumps must be of high precision and thus are very expensive. Typically, each such pump includes two displacement chambers, or syringes, that are operated approximately 180.degree. out of phase so that one pump chamber is discharging fluid while the other is charging. This category of high pressure system has a significant operating advantage, however, in that the component concentration in the output flow can be smoothly varied over time since each of the component pumps can have their flow rate independently adjusted.
The second category of mixing pump systems is termed a low pressure type. In this type, a single pump is utilized with each of the solvents, or other liquid components, connected from their separate sources to the input of the pump. Dual displacement chambers, or syringes, are generally used to make up this pump. Each liquid component supply is connected to the intake of the pump through a control valve. The pump is operated so that when a displacement chamber is being charged, each liquid component control valve is turned on for a portion of the charging part of the cycle. That is, in the usual case of a piston type displacement chamber, the valve of one constituent supply is opened for a first part of the intake stroke of the piston, it is then closed, and a valve of a second liquid component supply is opened for a time, and so forth, for each of the components that are to be mixed. While one displacement chamber is being charged with a mixture of components in a particular proportion, the other displacement chamber is discharging its previously mixed liquid. The two pistons of the two displacement chambers are mechanically connected to operate essentially 180.degree. out of phase with each other. The result is a liquid output whose composition changes in a step function fashion; that is, when the particular mixture in one chamber is completely discharged, the next chamber begins to discharge a mixture which usually is different in its percentage of liquid components. Thus, there is a sudden change in the proportion of components of the mixture when the discharge of one pump chamber ends and the other begins.
An advantage of the low pressure category of mixing pump systems is that only one pump with a pair of displacement chambers is required, in combination with appropriate intake valving of the various liquid component supplies, thereby minimizing the complexity and cost of a high precision pumping system. A disadvantage, however, of this category of mixing pumps, is its limited output flow rate range. The maximum flow rate is limited by the switching speed of the component liquid intake valves. As the output flow rate increases, the speed of the pump has to increase, which results in the intake stroke time being reduced. At some point, the intake stroke time becomes too short for the inlet valves to switch with enough control to provide the precise proportions of components desired during the short intake stroke.
The low pressure mixing systems also have a lower limitation of output flow. As the flow rate of such a pumping system decreases, the time for the delivery stroke becomes very long. Since the composition of the various liquid components in the displacement chamber is not changed during each discharge stroke, the pump system output composition has to remain the same for the duration of the delivery of each discharge stroke. The slower the pump operates, the longer is the duration of discharge of a particular fixed mixture. Since such a mixing pump system is usually utilized where the output mixture is desired to have a gradual change in relative concentrations of its components, there is some limit as to how long this period of constant output mixture can be tolerated. Thus, this places a lower limit on the flow rate available from the low pressure category of pumps.
Some improvements have been made in the operation of both categories of pumps by paying attention to the mechanical drive interconnection between the pistons of the two displacement chambers. Rather than operating exactly 180.degree. out of phase with the other, such linkages have been provided so that the common motive source of the pump provides a slightly longer discharge time of each chamber than its intake time. The reason for this is to smooth out the liquid flow rate by avoiding any discontinuities when one displacement chamber ends its discharge stroke and the other begins. In many applications, such as in the liquid chromatograph, a flow of uniform velocity and pressure is quite important. But such improvements have not addressed the more basic limitations of such pumping systems that are described above.
Therefore, it is a primary object of the present invention to provide a fluid mixing pump system that is mechanically simple, and thus of a lower cost, but which provides a liquid output that smoothly and accurately varies the composition of the fluid output over a wide range of pump output flow rates.
Another object of the present invention is to provide a mixing pump system that may be adapted for a wide variety of specific applications and requirements.